just god rays, without shadows or anything? (Blender Internal)
You could


Uncheck 'Specular' and 'Diffuse' illumination options in your spot lamp and only check 'Halo' (+ adjust 'Size', 'Intensity', etc. as you like)



To get some more defined and realistic rays you can add a so called cookie ("stencil") object (e.g. a plane with some holes cut in it). To make it just cast shadows and not show up in the render itself you have set up the "Shadow" section of its material as follows (main part being the "Cast Only" setting):

Use the Compositor node 'Filter'->'Sun Beams' to get the effect in post